A proximity sensor is a type of sensor that is configured to detect a presence or an absence of a target within an operational area of the sensor. There are different types of proximity sensors available. For example, the operation of the proximity sensors may be based on a capacitance between the sensor and the target or an inductance between the sensor and the target. Further, photoelectric sensors may also be considered as proximity sensors as a reflection of a transmitted light may be monitored and analyzed in order to detect the presence or the absence of the target. Also other types of proximity sensors are known. A selection of the type of proximity sensor is typically dependent on an application area in which the proximity sensor is applied to. More specifically, the selection depends heavily on a material of the target. As an example, if the target is metal an inductive proximity sensor is applicable.
The proximity sensor, and especially the inductive proximity sensor, comprises an inductive coil made of numerous turns of conductive wire, such as copper, and a capacitor for storing electrical charge. An input current is provided to an oscillator that generates an alternating current to the coil, which, in turn, generates a magnetic field in front of the proximity sensor. Now, when a target made of conductive metal is brought in a zone defined by boundaries of the magnetic field, some of the energy is transferred into the target causing eddy currents flowing in the target surface. Thus, the power loss affects to current flow in the internal LC resonance circuit of the proximity sensor, and when the target moves away from the zone at some point the state of the sensor changes. In other words, the sensor may indicate the presence of the target within the magnetic field for example by outputting a signal and when the target moves enough away from the boundaries of the magnetic field, the proximity sensor changes its state and the output signal is not present anymore.
The point at which the proximity sensor changes its state is known as a switching point. The switching points together form a three dimensional surface that defines a layer in space on which the output of the proximity sensor changes its state. Temperature has impact on a position of a switching point within the space. The switching point, and thus the switching point surface, may move farther or closer to the proximity sensor when the temperature of the sensor changes. This may cause problems when the proximity sensor is used in applications where the target movement is small. This is because the temperature change effect to the switching points has big impact to a total design tolerance chain. For example, proximity sensors applied in a hoisting motor brake of elevators is one environment in which the changes in temperature need to be taken into account as the movement of the target, i.e. an actuator, is very small and an allowance for tolerances in the design is tight. Moreover, the temperature may change a lot due to heavy forces applied in the motor.
In addition to the change in a position of the switching point in the proximity sensor the temperature change has also effect to volumes of parts being in the area of temperature change due to thermal expansion. Hence, both elements in the proximity sensor and the target, among any other, may change their volume along the temperature changes. Hence, the thermal expansion and the switching point temperature drift together cause problems especially in application areas wherein the distance between the proximity sensor and the target is small and the temperature may change a lot.
Hence, there is need to develop solution by means of which it is possible, at least partly, to mitigate an effect of temperature change in an application area of the proximity sensor when applied in a machinery brake of an elevator.